This invention relates generally to the automatic control of actuation force of a rapidly linearly displaced rod or piston. While not so limited, it is particularly useful in apparatus for the determination of the entrained gas-phase content of liquids. Apparatus of this type can determine the volume of entrained gas by rapidly actuating and impacting the liquid with a piston so that the pressure increase against the piston is indicative of gas phase content
Many test instruments and other devices make use of a rapidly accelerating force which acts against some material in order to measure a particular property. It is essential that acceleration of the device imposing the force should be consistent and reproducible. Maintaining consistent acceleration over time has proved to be a problem for various reasons. Among these can be mentioned wear of component parts or friction caused by small bits of trash being captured in seals.
Entrained gas-phase content in liquids can be measured by various direct and indirect methods. Some indirect methods include density, viscosity, and attenuation of sound waves. In general, indirect methods suffer limitations due to contributions by other factors. For example, density also depends upon general composition, and attenuation of sound waves depends upon the presence of suspended solids. Direct measurement can rely upon fluid compressibility. Simply stated, liquids are incompressible while gases are compressible. Methods relying upon fluid compressibility generally require the collection and isolation of a sample of liquid containing entrained gases. An exception to this rule is found in an earlier patent of the present inventor, U.S. Pat. No. 5,932,792. This patent describes apparatus for determining the gas content by impacting the liquid with a piston and measuring the pressure against the piston as indicative of gas content. This apparatus requires high-speed linear actuation in order to move the piston into the fluid with sufficient velocity so that the fluid adjacent to the piston, by virtue of its inertia, does not have time to move away from the end of the piston. The fluid adjacent to the end of the piston therefore serves as a dynamic containment vessel.
Actuators designed to produce linear motion experience changes in their linear translation behavior due to mechanical hysteresis and aging effects. For example, the magnetostrictive actuators described in my earlier patent utilize the change in length of a crystalline structure subjected to an external magnetic field. The migration of the microcrystalites within the crystal structure is actually not a smooth and reproducible phenomenon. Small changes in the movement pathway of the microcrystallites throughout the crystal structure result in slightly different elongation and relaxation characteristics from cycle to cycle, and over a multitude of cycles. Therefore, the actuator does not always produce the same linear translation when subjected to a given magnetic field. Other factors can influence the magnetic field itself, such as aging of the coil used to generate the magnetic field. Similar kinds of effects occur with other actuators such as piezoelectric and solenoid actuators.
In addition to changes in the linear translation characteristics of actuators, the apparatus described in my earlier patents is affected over time by other factors, such as seal friction, that can hinder piston movement to greater or lesser degrees over time.
Therefore, because a constant applied motivational force to an actuator incorporated within my earlier apparatus may not provide an identical linear displacement characteristic of the impacting piston from cycle to cycle, the pressure measured at the end of the impacting piston, at an otherwise constant entrained gas content, may drift over time.
The present invention describes a method for overcoming this problem. However, it is more broadly useful for automatic control of the intensity and acceleration of the pulsed rapid linear movement of any similar device.
I have now discovered that in an apparatus of the type described in my earlier patent, the magnitude of the pressure pulses correlate closely with the acceleration of the impacting piston at a constant entrained gas content. Therefore, independent control of the acceleration of the impacting piston will ensure that reproducible pressure pulses are obtained at a given constant entrained gas content. The acceleration can be measured directly with an accelerometer. Alternatively, the acceleration can be calculated from the change in linear displacement over time since the second derivative of displacement with respect to time is identical to acceleration.
The acceleration produced during an actuator cycle could be expected to proceed as follows. Initially, when the actuator is at idle, there is zero acceleration. When the actuator begins to move the impacting piston into the fluid there is an increase in acceleration. When the linear movement of the impacting piston begins to decrease to zero, the acceleration decreases and approaches zero. As the impacting piston is retracted, the acceleration is negative. Finally, when the impacting piston has again come to rest, the acceleration becomes zero. As has been observed in actual practice, the characteristics of the acceleration are much more complicated than given by the preceding description. The acceleration may actually go through several positive peaks before the acceleration becomes negative. The complex behavior in the acceleration of the impacting piston results in a multiplicity of pressure responses at the end of the impacting piston.
The acceleration which occurs early in an impact cycle has the most important influence on the resulting pressure pulses even though subsequent acceleration may be greater than the early acceleration. Whereas the inertia of the fluid adjacent to the end of the piston causes the fluid to serve as a dynamic containment vessel, this effect will diminish rapidly as the fluid does begin to move under the action of the piston. Therefore, the first pressure pulse is most indicative of entrained gas content, while subsequent pressure pulses reflect a combination of compression of the fluid and simple inertial acceleration of the bulk fluid. Emphasis is therefore placed on the measurement of the early acceleration for the purpose of controlling the ability of the impacting piston to measure the entrained gas content.
The preferred method to measure the acceleration is with a dedicated accelerometer. Alternatively, a linear displacement transducer can be used to measure the position of the impacting piston over time, and acceleration can be calculated from the shape of the displacement-time curve.
A principal object of the present invention is to provide a method and apparatus for the automatic control of the amount of activation energy applied to a linear actuator so that the resultant acceleration will exhibit minimal drift over time.
Another object is to provide a method and apparatus for automatic control of activation energy applied to a rapidly translating piston used to measure entrained gas within a liquid.
A further object is to combine multiple acceleration responses in order to produce a single representative value of acceleration for control of energy applied to the actuator.
It is an additional object of the invention to provide a method and apparatus to control the performance of an actuator by measuring the acceleration of an impacting piston, and to use that information to automatically control the actuator as necessary to maintain constant acceleration.
These and many other objects will become readily apparent upon reading the following detailed description, taken in conjunction with the drawings.